The tax system allowed (and continues to allow) married couples to split their income for tax purposes, which acts as a strong disincentive to wives going out to work.
In separate work, equally unpalatable to many people, Mr Levitt shows that juveniles respond to the disincentive effects of punishment in much the same way that adults do.
In this case, low tax rates are creating a disincentive to take on investment risk and is allowing my friend to play golf rather than return to the work force.